Process for coating zinc and zinc-iron



United States Patent 3,127,288 PROCESS FOR CGATING ZINC AND ZINC-IRON ALLOY SURFACES William G. Hines, Wiilowdale, Ontario, and Ramon L. Addinail, Stoney Creek, Ontario, Carlada, assignors to The Steel Qompany of Cant-gig, Limted, Hamilton, Ontario Canada, a company 0 ana a N0 Drawing. Filed May 26, 1960, Ser. No. 31,819 7 Claims. (Cl. 148-6.16)

This invention relates to a method of treating zinc and zinc-iron alloy surfaces to protect them from corrosion and staining, and zinc and zinc-iron alloy surfaces protected thereby.

It is known that products having exposed surfaces formed of zinc and zinc-iron alloys tend to corrode and discolour, particularly when exposed to n1oisture or to humid atmospheric conditions while in storage and in use. This is particularly evident in the formation of what is known in the art as white rust on surfaces of galvanized sheets, particularly when stored under moist or humid atmospheric conditions. Methods are known for applying protective coatings to zinc and zinc-iron alloy surfaces. While these coatings have been generally effective for the protection of zinc surfaces against corrosion, none has been completely satisfactory for the treatment of zinc and zinc-iron surfaces.

An important object of this invention is to provide a method for treating zinc and zinc-iron alloy surfaces to protect those surfaces against corrosion or discolouration without adversely affecting the appearance, solderability or the paintability of the treated surface.

The object of this invention can be attained by treating the zinc or zinc-iron alloy surfaces, prior to storage or use, with an aqueous solution which contains phosphate, (P), ions and at least one member of the group consisting of chromate, (CrO ions and nitrite, (N05),

ions.

The source of phosphate, (P05), ions used in this process is, preferably, commercial grade phosphoric acid, about 85% by weight concentration. However, other oxyacids of phosphorus and salts thereof can be substituted for all or part of the phosphoric acid. The phosphate ions are provided in the treating solution in amount equivalent to from about 0.5% to about 3%, preferably from about 1% to 2%, by weight of phosphoric acid. The oxyacids of phosphorus include, but are not necessarily limited to, orthophosphoric acid, pyrophosphoric acid and metaphosphoric acid. The salts of these acids usually will be the sodium and potassium salts but other salts can be employed if desired.

The source of the chromate, (cro,-), ions is preferably chromic acid or a salt thereof, provided in the treating solution in amount equivalent to from about 0.5% to about 3%, preferably from about 1% to 2%, by weight of chromic acid. Salts of chromic acid used in this process usually will be the sodium or potassium chromate or dichromate. Of these, the potassium salts are preferred. Nitrite, (NO ions such as in the form of sodium nitrite, in amount of from about 0.1% to about 0.4% can be substituted for all or part of the chromate ions. The surfaces of articles treated with a solution which contains a higher concentration of nitrite ions tend to discolour in storage.

The pH value of the solution should be maintained within the range of from about pH 1.3 to about pH 3.8, preferably from about pH 1.6 to pH 2.4, during its use as a coating solution. We have found that when the treating solution has a pH below about 1.3, operating difiiculties are encountered due to stickiness of the treated surfaces, although the coating itself is satisfactory. It is found that at a pH above about 3.8, reagents tend to precipitate from the solution, at least in part, as a chromium phosphate compound.

It is found, also, that continued dipping of zinc or zinciron alloy surfaced sheets or articles into a bath of solution tends to neutralize the acidity of the solution, and thus increase its pH value above the desired maximum. Replenishment of the solution with phosphoric acid and chromic acid tends to reduce the pH value below the desired minimum. It is therefore preferred to maintain the pH value of the bath of solution during its use by the addition of phosphoric acid and a chromate salt, such as the dichromate, or a mixture of phosphoric acid, chromic acid and a chromate salt, which serves to maintain the desired constitution of the bath and the pH value within the desired range without reducing the pH value below the desired minimum.

It will be understood that the treating solution can be prepared from a large number of combinations of phosphoric and chromic acids and salts, and other acids, such as nitric acid, can be used to adjust the pH value without harmful effect on the resulting products. From the economic and operating points of view, orthophosp'horic acid, potassium dichromate and chromic acid is the preferred combination.

We have found, also, that the best results are obtained from the coating when the solution is applied to the surface being treated at a temperature of from about 70 F. to about 170 F., with a preferred temperature of about F.

The coating treatment is very easily conducted. The articles having exposed surfaces formed of zinc or a zinciron alloy, such as galvanized sheets, can be dipped into the solution, excess solution removed, and the article thereafter dried, such as by heating or by air drying. Alternatively, the treating solution can be applied by spraying. In treating galvanized sheets, a satisfactory procedure is to dip the sheets in the solution, or spray them, remove excess solution by passing them through wringer rolls, and thereafter dry them, such as by heating or air drying, to remove residual moisture. The articles can then be stored, regardless of atmospheric conditions, without danger of corrosion or discolouration.

The process is, of course, based on actual results obtained from extensive tests and investigations and is not dependent upon hypothetical considerations. The addition to the phosphate ion, such as in the form of phos phoric acid, an oxyacid of phosphorus, or a salt thereof, is believed to be of primary importance in the solution. However, it is found that the addition of the phosphate ion alone may result in the formation of random corrosion spots at air-water interfaces. The addition of the chromate ion, such as in the form of chromic acid or a salt thereof, is effective in preventing the appearance of random corrosion spots. Sodium nitrite, in combination with the phosphate ions, also is effective in preventing the formation of random corrosion spots on the zinc and zinc-iron alloy surfaces at air-water interfaces but is not as effective as chromic acid or a salt thereof.

Articles having zinc or zinc-iron alloy surfaces, such as galvanized sheets, treated by the process of this invention, have been tested under severe atmospheric conditions over long periods of time without corrosion or staining, whereas untreated surfaces exposed to the same conditions showed marked staining and corrosion in less than 10 hours. Surfaces treated by known solutions exposed to the same atmospheres showed definite staining and corrosion within two days.

What We claim as new and desire to protect by Letters Patent of the United States is:

1. The process of treating zinc and zinc-iron alloy surfaces which comprises treating said surfaces with an aqueous solution having a pH value within the range of a from about pH 1.3 to about pH 3.8, said solution consisting essentially of water, phosphate ions in amount equivalent to from about 0.5% to about 3.0% by weight phosphoric acid and at least one compound selected from the group of compounds containing chromate ions in amount equivalent to from about 0.5 to about 3.0% by weight chromic acid and of compounds containing nitrite ions in amount equivalent to from about 0.1% to about 0.4% by weight sodium nitrite.

'2. The process according to claim 1 in which the phosphate ions are provided in said solution by the addition thereto of a member selected from the group consisting of a phosphoric acid and a salt thereof.

3. The process according to claim 1 in which the phosphate ions are provided in said solution by the addition thereto of a member selected from the group consisting of a phosphoric acid and a salt thereof and the chromate ions are provided in said solution by the addition thereto of a member selected from the group consisting of chromic acid and a salt thereof.

4. The process according to claim 1 in which the solution is applied to the surfaces at a temperature within the range of from about 70 F. to about 170 F.

5. The process according to claim 1 in which the pH,

value of the solution is maintained during the treatment of zinc and zinc-iron alloy surfaces within the range of from about pH 1.3 to about pH 3.8 by the addition thereto i 41 of phosphoric acid and a member selected from the group consisting of chromic acid and chromate salts thereof and sodium nitrite.

6. The process for treating zinc and Zinc-iron alloy surfaces which comprises the steps of coating said sheets with an aqueous solution having a pH value Within the range of from about pH 1.3 to about pH 3.8 and which consists essentially of water, a compound selected from the group consisting of phosphoric acid, oxyacids of phosphorus and salts thereof in amount equivalent to from about 0.5% to about 3.0% by weight phosphoric acid and a compound selected from the group consisting of chromic acid and chromate salts thereof in amount equivalent to from about 0.5 to about 3.0% by weight chromic acid, and sodium nitrite in amount within the range of from about 0.1% to about 0.4% by weight, and removing excess solution from and drying said sheets.

7. The process according to claim 6 in which the solution is applied to the surfaces at a temperature within the range of from about F. to about F.

References Cited in the file of this patent UNITED STATES PATENTS 2,272,216 Lodeesen Feb. 10, 1942 2,497,905 Ostrander Feb. 21, 1950 2,766,153 Russell Oct. 9, 1956 2,891,884 Rausch et al June 23, 1959 

1. THE PROCESS OF TREATING ZINC AND ZINC-IRON ALLOY SURFACES WHICH COMPRISES TREATING SAID SURFACES WITH AN AQUEOUS SOLUTION HAVING A PH VALUE WITHIN THE RANGE OF FROM ABOUT PH 1.3 TO ABOUT PH 3.8, SAID SOLUTION CONSISTING ESSENTIALLY OF WATER, PHOSPHATE IONS IN AMOUNT EQUIVALENT TO FROM ABOUT 0.5% TO ABOUT 3.0% BY WEIGHT PHOSPHORIC ACID AND AT LEAST ONE COMPOUND SELECTED FROM THE GROUP OF COMPOUNDS CONTAINING CHROMATE IONS IN AMOUNT EQUIVALENT TO FROM ABOUT 0.5 TO ABOUT 3.0% BY WEIGHT CHROMIC ACID AND OF COMPOUNDS CONTAINING NITRITE IONS IN AMOUNT EQUIVALENT TO FROM ABOUT 0.1% TO ABOUT 0.4% BY WEIGHT SODIUM NITRITE. 